Known as carboxyl group-containing water-soluble polymers are, for example, copolymers of an α,β-unsaturated carboxylic acid such as acrylic acid and a polyallyl ether (cf. e.g. Patent Document 1), copolymers of an α,β-unsaturated carboxylic acid and hexaallyl-trimethylene-trisulfone (cf. e.g. Patent Document 2), copolymers of an α,β-unsaturated carboxylic acid and triallyl phosphate (cf. e.g. Patent Document 3), copolymers of an α,β-unsaturated carboxylic acid and glycidyl methacrylate or the like (cf. e.g. Patent Document 4), an α,β-unsaturated carboxylic acid such as acrylic acid and a pentaerythritol allyl ether (cf. e.g. Patent Document 5; Patent Document 6 and Patent Document 7) and copolymers of an α,β-unsaturated carboxylic acid such as acrylic acid, a (meth)acrylic acid alkyl ester and a pentaerythritol allyl ether (cf. e.g. Patent Document 8), among others. These carboxyl group-containing water-soluble polymers are used, after dissolution in water and neutralization with an alkali to give viscous liquids, in such fields of application as thickening agents for cosmetics and the like, humectants for cataplasms and the like, and suspension stabilizers for emulsions, suspensions and the like, among others.
In using the carboxyl group-containing water-soluble polymers mentioned above in these fields of application, it is necessary to first prepare a uniform aqueous solution of any of the carboxyl group-containing water-soluble polymers and then neutralize the same to give a neutral viscous liquid. However, the carboxyl group-containing water-soluble polymers mentioned above each generally occurs as a fine powder and, therefore, undissolved lumps are readily formed on the occasion of dissolution thereof in water. Once undissolved lumps are formed, a gel-like layer is formed on the surface of each lump, so that the rate of penetration of water into the inside thereof is reduced and it becomes difficult to obtain a uniform aqueous solution; this is a drawback.
Therefore, in preparing an aqueous solution of a carboxyl group-containing water-soluble polymer, a procedure consisting in gradually adding the carboxyl group-containing water-soluble polymer powder to water with high-speed stirring is required; this procedure is poor in production efficiency and, in some instances, a special dissolution apparatus is required for preventing the formation of undissolved lumps. Furthermore, from the production efficiency improvement viewpoint, it is a recent trend in dissolution technique to disperse and dissolve the carboxyl group-containing water-soluble polymer in advance at a high concentration and then dilute the resulting solution for use.
On the other hand, in the field of cosmetics and the like, there is a tendency toward decreasing the carboxyl group-containing water-soluble polymer addition level to improve the texture of cosmetics and the like. Therefore, a carboxyl group-containing water-soluble polymer capable of giving a neutralized viscous aqueous solution excellent in thickening effect at lower concentrations is desired.    Patent Document 1: U.S. Pat. No. 2,923,692    Patent Document 2: U.S. Pat. No. 2,958,679    Patent Document 3: U.S. Pat. No. 3,426,004    Patent Document 4: Japanese Kokai Publication S58-84819    Patent Document 5: U.S. Pat. No. 5,342,911    Patent Document 6: U.S. Pat. No. 5,663,253    Patent Document 7: U.S. Pat. No. 4,996,274    Patent Document 8: Japanese Kokai Publication S59-232107